Nickeliferous oxide ores, i.e. nickeliferous limonites and nickeliferous silicates, are the world's largest potential sources of nickel and cobalt. The inability to beneficiate these ores by conventional techniques, such as froth flotation or magnetic separation, has placed these ores at an economic disadvantage as compared to sulfide ores which can be concentrated by well known means.
One process for recovering nickel and cobalt from oxide ores is that practiced in Cuba at Moa Bay. The Moa Bay process involves acid leaching at elevated temperatures sufficiently high that the sulfates of aluminum and iron are substantially insoluble. A recurring problem with this process is the build-up of scale on all reactor parts exposed to the acidified slurry. As the scale builds up, the capactiy of the autoclave is lowered and eventually processing must be terminated for scale removal.
IN U.S. Pat. Nos. 3,773,891 and 3,809,549 and in U.S.S.R. Pat. No. 256,264 there are disclosed processes for leaching nickel from oxidic ores by oxidizing pyrites to form sulfuric acid in situ. In order to take advantage of the exothermic nature of the pyrites oxidation, the slurry of nickeliferous oxide ore and pyrites is preheated to a temperature sufficient to initiate the oxidation reactions and the slurry is thereafter heated to the leaching temperature by the heat released by the oxidation of pyrites. The U.S. patentees state that the comparatively mild leaching conditions reduce scale formation. The effective increase in capacity realised by extending the time between shutdowns for scale removal is more than offset, however, by the increase in residence time that is required for the complete oxidation of pyrites and the effective decrease in capacity by the pyrites displacing a portion of the ore. Other drawbacks of these processes are the additional amount of piping and control mechanisms that are required to maintain proper partial pressures of oxygen for pyrite oxidation. The additional piping and control devices increase both capital and operating costs. An even further problem with these processes is that in order to maintain your requisite partial pressures of oxygen the atmosphere over the slurry must be continuously exhausted so that even though the oxidation of pyrites is exothermic the need for exhausting the atmosphere renders the overall process endothermic.